Ditto with both the above. Its not only personal preference but some have other commitments such as other jobs/kids, some may have medical issues meaning they can't physically see many clients, even if they wanted to.
And not forgetting competition, some areas are more heavily populated with wgs than others. Last but not least you could get 2 wgs of the same age, who offer a similar service, in the same area and yet one may do brilliantly and the other, not well at all.